1. Field of the Invention
The present invention generally relates to the methods of batch processing powder-metallurgy (P/M) aluminum alloy and metal matrix composite (MMC) sheet and plate products. More particularly, the present invention relates to the method of manufacturing such product at yield rates higher than conventional batch processes.
2. Description of the Prior Art
Powder-metallurgy MMC's are advanced composite materials that are made from metal powders and reinforcement materials by powder-metallurgy manufacturing methods. P/M MMC can be produced in many forms, such as extruded tubes and rolled sheets.
P/M MMC sheets have many applications in aerospace such as skins and control surfaces, and in the nuclear industry as neutron absorbing material. Each of these applications uses MMC for its special properties which are controlled by different MMC compositions. However, all P/M MMC sheet products produced by prior-art manufacturing methods suffer from low production yield rates from the initial cylindrical shaped billets.
Typical sheet yield-rates are approximately 30 to 60% from an initial MMC billet. The higher the percentage of reinforcement in the MMC, the lower the recovery rate for MMC sheet production. The low sheet yield rate reduces productivity and increases cost, and also generates large quantities of scrap. The scrap cannot be recycled as composite and must be recycled as dirty metal, which severely reduces the recovery price. The matrix metal is usually recovered. However, the expensive reinforcing material such as boron carbide is removed with the flux and discarded. As a result, low yield rate and high cost-of-production in current manufacturing processes have limited the applications of MMC sheets, even though they have properties superior to conventional metallic sheets.
There have been many efforts to make metal matrix composites, as demonstrated in, for example: U.S. Pat. No. 4,104,061 issued to Roberts; U.S. Pat. Nos. 4,557,893 and 4,623,388 issued to Jatkar et al.; U.S. Pat. No. 4,946,500 issued to Zendalis et al.; U.S. Pat. No. 4,722,751 issued to Akechi; U.S. Pat. No. 5,561,829 issued to Sawtell et al.; U.S. Pat. No. 5,965,829 issued to Haynes et al.; and U.S. patent application Ser. No. 60/387,781 filed by Harrigan et al. However, none of these works addressed the manufacturing problem of low yield rate for P/M MMC sheet production.
A typical prior-art manufacturing process for P/M MMC sheet production is illustrated in FIG. 1, often called the billet-extrusion-rolling (BER) processing. An initial cylindrical MMC billet is extruded into a rectangle-shaped workpiece followed by hot rolling to the desired thickness. The extrusion process is used not only to convert the billet into an easily rolled product form. It also imparts thermo-mechanical work to the MMC which improves its mechanical properties leading to improved rolling behavior.
The extruded rectangle workpiece possesses a nose defect at the lead end and pipe defect at the butt end. The nose defect manifests itself as a radiused leading edge that has little or no hot work and must be removed during the preparation of the roll preform. The pipe defect is a linear discontinuity that occurs because the center of the MMC billet flows faster than the outside due to friction between extrusion press container, the die surface and the MMC billet.
If a MMC workpiece is rolled without the nose defect removed, significant edge cracking may develop during the rolling process which reduces the recovery rate. If a MMC workpiece is rolled without the pipe defect removed, the final sheet product will contain a large delamination near the centerline of the thickness. Therefore, the nose and pipe defects must be removed from the extrusion in order to produce good quality sheet. Material losses associated with the extrusion process are about 15 to 18%.
Edge cracking of the MMC roll-preform occurs during the rolling process. Edge cracks are caused by the limited ductility of the MMC, high shear stresses in the work piece, and the low working temperature at the sheet edges. An edge crack can be as long as 15 cm (˜6″) from the rolled sheet edges. Edge cracking during the rolling process can represent approximately 28% of the weight of the MMC roll-preform, and they must to be removed from the final sheet product before delivery to the customer.
A “picture frame” rolling method is used to roll materials that have limited ductility. The method uses a metallic picture frame surrounding the material to be rolled. The picture frame and roll preform are rolled together as an assembly. The frame minimizes or prevents rolling edge cracks.
For example, U.S. Pat. No. 4,705,577 issued to Ondracek et al. and U.S. Pat. No. 4,634,571 issued to Langhans et al. described the picture frame method to roll nuclear fuel composites. The frame is retained as part of the final product.
A picture frame rolling method has been used for rolling aluminum MMC (Al-MMC) sheet. A machined aluminum frame was heated and an Al-MMC rectangular roll-perform was placed inside the frame. As the frame cooled, it shrank to a tight fit around the rectangle Al-MMC. The framed Al-MMC was heated and rolled with multiple passes on a rolling mill to final thickness. The aluminum frame did not retain a tight fit with the rectangular Al-MMC roll preform during all rolling passes because the frame was not metallurgically bonded with the Al-MMC. Therefore, edge cracks, although reduced in length, occurred during the rolling process.
There is also a “box-frame” method used to produce a MMC sheet. Blended boron carbide and aluminum powder is loaded to a welded aluminum box. The mixture is compacted at room temperature and an aluminum lid is welded in place to seal the box. The box-frame with the blended powder mixture is heated and then rolled to produce a MMC sheet with monolithic aluminum cladding sandwiched on either side of a MMC core that is not fully densified. Such sandwich sheet has poor mechanical and thermal properties and suffers from delamination and blistering problems in corrosive environments. As a result, sandwich sheets have limited applications.
Therefore, there is a real need to improve the manufacturing technology for MMC sheet, with the specific objective being improved yield rates.